I'd say that it has to do more with acclimation than anything else. On Earth we generally don't fall long enough to get used to the sensation. In orbit the body just adjusts to the new "norm" and eventually just ignores it.

how come when we fall near the earth, our stomachs lurch, but astronauts in their free-fall state out in orbit feel fine?

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Some astronauts do get space-sickness. Last I heard, in spite of a fair amount of research, nobody has been able to predict which ones will get space-sick and which one's won't. One might think that the short duration free-fall flights in the "vomit comet" would help screen out astronauts that were prone to space-sickness, but apparently this doesn't actually work.

nobody has been able to predict which ones will get space-sick and which one's won't. One might think that the short duration free-fall flights in the "vomit comet" would help screen out astronauts that were prone to space-sickness, but apparently this doesn't actually work.

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This might involve visual stimulae as well as physical situations. I, for instance, am absolutely terrified of heights but will do anything in an aeroplane. There are no reference lines to the ground, so it doesn't feel like being 'high'. Low-g in a plane feels perfectly natural (barely noticeable), but it bugs the hell out of me in an elevator or in those instances when my car leaves the ground briefly. I would expect that to be worse in a space-station environment, where every movement makes your body act like a gyroscope and things that should be on the ground are floating around your head.

how come when we fall near the earth, our stomachs lurch, but astronauts in their free-fall state out in orbit feel fine?

Click to expand...

I know the feeling you're talking about, and still get it if the car goes over a hump backed bridge, but I've been skydiving for 30 years, and have never suffered the same sensation, although 10,000 - 13,000 ft is still "near the earth" compared with astronauts.

If you're skydiving you should spend most of the time at terminal velocity and not accelerating, so you wouldn't get it as badly as an astronaut at least. Interesting that you've never felt it all though.